1. Field of the Invention
The invention relates to optical rotation rate sensors and gyroscopes, and more particularly to the design and fabrication of lock-in-free, monolithically integrated semiconductor unidirectional ring laser sensors/gyroscopes.
2. Discussion of Related Art
The Sagnac effect evolves from the interaction of light with rotational movement, and provides a means of detecting the rotation, its orientation, and angular speed (see a review paper “Sagnac Effect” by E. J. Post, Rev. Mod. Phys., vol. 39, pp. 475-493, 1967). Prior attempts to use semiconductor bidirectional ring lasers for achieving rotation or gyroscopic sensing, described for example in P. J. R. Laybourn, M. Sorel, G. Giuliani, and S. Donati, “Integrated Semiconductor Laser Rotation Sensor”, Integrated Optics Devices III, Jan. 25-27, 1999, San.Jose, Calif., Proc. SPIE, vol. 3620, pp. 322-331, have not resulted in detection of a Signac beat frequency due to a high lock-in rate, resulting from significant backscattering of counterpropagating light within the bidirectional structure.
To date, the Sagnac effect has not been realized in either unidirectional ring laser (URL) or bidirectional ring laser (BRL) semiconductor structures. This may be due to intrinsic limitations associated with previously applied gyro designs.
Research and development directed at fabrication of chip-size semiconductor ring lasers for application as rotation sensors has been quite limited and is still very immature. Early attempts to use semiconductor BRLs for the purpose of rotation sensing had been unsuccessful and were abandoned for many years. As a new development, monolithic integration of a ring laser, waveguides, couplers and a detector in a single semiconductor chip was reported in P. J. R. Laybourn, M. Sorel, G. Giuliani, and S. Donati, “Integrated Semiconductor Laser Rotation Sensor”, Integrated Optics Devices III, Jan. 25-27, 1999, San Jose, Calif., Proc. SPIE, vol. 3620, pp. 322-331, indicating that a new generation of chip-size rotation sensors may be evolving.
During the dynamic testing of an integrated semiconductor gyro, however, no beat frequency associated with Sagnac effect could be detected, even at rotation rates approaching Ω=4000 rad s−1. The absence of a beat frequency was attributed to a high lock-in rate resulting from significant backscattering of light within the BRL structure and from a variety of index discontinuities and transition regions within the integrated chip structure. The lock-in threshold rotational speed for an open-cavity ring laser is given by the expression                                           Ω            L                    =                                                    r                21                            ⁢              c              ⁢                                                           ⁢                              E                2                                                                    E                1                            ⁢              L                                      ,                            (        1        )            where r21 is the coupling coefficient between two counterpropagating modes of amplitudes E1 and E2, c is the speed of light in vacuum, and L is the ring cavity length. Experimental verification of the magnitudes of the variables shown in Eq. (1) as they relate to a closed loop (constant L) ring laser gyro is paramount to success in gyro design.
However, direct measurement of the variables is not possible using the BRL design of P. J. R. Laybourn, M. Sorel, G. Giuliani, and S. Donati, “Integrated Semiconductor Laser Rotation Sensor”, Integrated Optics Devices III, Jan. 25-27, 1999, San Jose, Calif., Proc. SPIE, vol. 3620, pp. 322-331, rendering the exact mechanism(s) responsible for the reported lock-in as indeterminate. Moreover, it has not even been established that bidirectional operation, essential for rotation sensing in a BRL gyro design, has actually been achieved. In fact, M. Sargent in “Theory of a Multimode Quasi-Equilibrium Semiconductor Laser”, Phys. Rev. A, vol. 48 (1), pp. 717-726, July 1993. predicted theoretically that stable single-frequency BRL operation could not even occur in semiconductor lasers.
Most recent measurements of the output signals from BRLs by M. Sorel, P. J. R. Laybourn, G. Giuliani, and S. Donati, reported in “Unidirectional Bistability in Semiconductor Waveguide Ring Lasers”, Appl. Phys. Lett., vol. 80, pp. 3051-3053, 2002, confirmed that this prediction was indeed correct, and rather than a stable bidirectional emission, a bistable unidirectional ouptut was observed
It is desired to achieve the Sagnac effect in semiconductor ring lasers. It is further desired to provide a ring laser gyroscope that greatly reduces or eliminates the lock-in phenomenon associated with bidirectional ring lasers and other gyro designs.